1. Field of the Invention
This invention relates to a highly efficient coding apparatus for a picture signal, such as a television signal, and, more particularly, is directed to a coding apparatus in which a subsampling and a block coding, such as a coding adaptive to a dynamic range or ADRC, are both employed.
2. Description of the Prior Art
In the transmission of a digital video signal, subsampling is a known method for compressing an amount of data, as compared with the original amount, and is the thinning-out of picture elements through subsampling by lowering the subsampling frequency. One proposed subsampling method for thinning-out picture data to 1/2, is to transmit a two-bit flag, indicating the position of a subsampling point, and that subsampling point is used in the decoding at the time of interpolation of thinned-out picture elements. For example, if the picture element data of a digital video signal is eight bits and a two-bit flag is added, then the bit number per picture element becomes five bits and the compression rate is (5/8).
Since a subsampling pattern, for instance a line-quincunx, is always fixed in this conventional subsampling, there is the problem that the quality of a reproduced picture is substantially deteriorated in a portion of a contour of an object in the picture. Particularly, when the subsampling period is lengthened more than 1/2, the deterioration of the picture quality is substantial.
To solve the above-mentioned problem, the present applicant has proposed an encoding method as described in the specification of the Japanese Patent Laid Open Publication (JP, A) No. 266924/1987. In this method, a picture is divided into a number of two-dimensional blocks, the difference (dynamic range) between the maximum value and the minimum value of plural picture element data in the block is obtained, and the period of the subsampling is varied in dependence on the dynamic range of the blocks. More specifically, if a block has a small dynamic range, it is deemed to represent a planar picture, and the subsampling period is increased, for example to (1/8). On the other hand, if a block has a comparatively large dynamic range, it is deemed to represent a changing picture, and the subsampling period is reduced to (1/2). Further, if a block has an extremely large dynamic range, then it is deemed to represent a substantially changing picture, and no subsampling is applied.
Since the subsampling period is set on the basis of the block as a unit, in a highly efficient coding apparatus, the switching of the subsampling frequency is selectively dependent on a dynamic range, as described above. The picture quality of a reproduced picture, whether it is good or poor, necessarily occurs at a unit of block and, for this reason, substantial block distortion appears. Further, the dynamic ranges of blocks to be selected as subsampling periods are limited, so that the adaptability of the subsampling period to a characteristic of a picture may be insufficient.
The present applicant has further proposed a highly efficient coding apparatus for a picture signal which does not cause the above described deterioration of the block unit and which can form a subsampling pattern having a variable density adaptive to a characteristic of a picture so as to provide excellent picture reproduction (for example, as disclosed in the specification of the Japanese Patent Application No. 208957/1987).
Another highly efficient coding apparatus for a picture signal which has the same advantage as the invention disclosed in said specification of the immediately above identified Japanese Patent Application and which uses real data at the time of calculation of interpolation errors to enable real-time processing and to be applicable to an animation, has been proposed by the present applicant (for example, in the specification of the Japanese Patent Application No. 85210/1987).
In the above-proposed variable density subsampling, since the thinning-out process is performed adaptively, the generated data amount varies greatly depending on the degree of correlation of a picture. As a consequence, problems may arise in that the transmission data amount may not exceed a predetermined required minimum value or by reason of the fact that it is applied to a transmission path (for example, a digital VTR) which requires an almost constant data rate. Further, in the case of a small transmission capacity, the reduction of the amount of data only by subsampling may not be sufficient.